1. Field of the Invention
The present invention relates to an improvement in or relating to a machine which cuts and shapes the inner surface of a given work piece such as a differential gear used in an automobile.
2. Description of the Prior Arts
FIG. 7 shows a conventional inner surface shaper. It comprises, on a bed 10, left and right reciprocating stocks 63 and 64 each carrying a working spindle 61 or 62 and a motor (not shown), and associated reciprocating drives 69 and 70 each having a hydraulic cylinder 65 or 66 to make a linear reciprocating motion of the reciprocating stock along an associated drive screw 67 or 68, thereby permitting these reciprocating stocks 63 and 64 to move toward and apart from each other. In operation the reciprocating drives 69 and 70 drive the reciprocating stocks 63 and 64 to move toward each other until a tool 73 has been sandwiched between the opposite tool supports 71 and 72 of the working spindles 61 and 62 for rotating. The hydraulic cylinder 65 drives the reciprocating stock 63 toward a stopper 74 to cut and shape the inner surface of the work piece 75.
The feeding of the tool 73 by the hydraulic cylinder 65, however, is liable to be dependent on the temperature of the oil in the cylinder, lowering the accuracy with which the cutting-and-shaping is effected. Also disadvantageously, the accurate setting of feeding distance is hardly attainable with recourse to the positioning of the stopper 74.
In an attempt to reduce such disadvantages the inventor proposed that the conventional inner surface shaper is modified by using, in place of the reciprocating drive 69, a drive screw extending under the reciprocating stock 63 and threadedly engaged with a nut which is fixed to the bottom of the reciprocating stock 63 and by using a servomotor to drive the drive screw for making a linear reciprocation of the reciprocating stock 63. In operation the tool is put in position by driving the reciprocating stock 63 with the aid of the nut-and-screw drive, and then the hydraulic cylinder 66 pushes the tool support 72 against the tool, which is fed by the nut-and-screw drive. This arrangement permits the accurate setting of feeding distance, and accordingly the accuracy with which the tool can give the work piece a desired shape is improved.
In this improved inner surface shaper, however, the pushing and supporting of the tool 73 by the hydraulic cylinder 66 and the feeding of the reciprocating stock 63 by the servomotor through the nut-and-screw drive will cause counter force to push the opposite reciprocating stocks 63 and 64 away from each other, thus causing a distortion in the opposite reciprocating stocks. Therefore, the machining accuracy is limited. Such a distortion causes slight vibration in machining, the bending of the screw drive or uneven wearing of parts which move relative to each other. For these reasons the machining accuracy cannot be retained for an extended period.